ABSTRACT
The properties of virtually all real-world
materials change with time, causing their BRDFs to be
time-varying. However, none of the existing BRDF models and
databases take time variation into consideration; they
represent the appearance of a material at a single time
instance. In this work, we address the acquisition,
analysis, modeling and rendering of a wide range of
time-varying BRDFs. We have developed an acquisition system
that is capable of sampling a material¡¯s BRDF at multiple
time instances, with each time sample acquired within 36
seconds. We have used this acquisition system to measure the
BRDFs of a wide range of time-varying phenomena which
include the drying of various types of paints (watercolor,
spray, and oil), the drying of wet rough surfaces (cement,
plaster, and fabrics), the accumulation of dusts (household
and joint compound) on surfaces, and the melting of
materials (chocolate). Analytic BRDF functions are fit to
these measurements and the model parameters¡¯ variations with
time
are analyzed. Each category exhibits interesting and
sometimes non-intuitive parameter trends. These parameter
trends are then used to develop analytic time-varying BRDF (TVBRDF)
models. The analytic TVBRDF models enable us to apply
effects such as paint drying and dust accumulation to
arbitrary surfaces and novel materials.

Time-Varying BRDF (TVBRDF) Database--NewWe have acquired a variety of
samples including watercolors, spray paints, oil paints,
fabrics, cement, clay, plaster, joint compound dust,
household dust and chocolate. A complete list of our 41
samples and the models used to fit their data can be found
on the project page.

Our goal is to use this database to first identify temporal
trends in the estimated parameter values that are associated
with each type of time-varying phenomenon (drying paint,
drying wet surface, dust accumulation). Next, we propose
analytic functions that model these temporal trends in
parameter values. These models enable us to apply several
of the above physical processes to novel materials.